Audible fire alarm



June 23, 1970 F. s. STEIN ETTAL AUDIBLE FIRE ALARM Filed Aug. 9, 1965 IN VENTORS 6 Y a. W Z m 3 T A 5&7 w WM 0 United States Patent 3,517,382 AUDIBLE FIRE ALARM Frank S. Stein and Robert E. Knolinski, Kokomo, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Aug. 9, 1965, Ser. No. 478,328 Int. Cl. B60q /00; G08b 3/10, 17/06 US. Cl. 34057 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a temperature sensitive warning system which utilizes a temperature sensitive semiconductor element and other components conventionally used in a given installation.

There are often components used in various installations that would give a visual or audible Warning signal, such as lights or devices providing sound. For example, audible horns are normally provided as warning devices for manual driver operation in automobiles. Such a device could be used together with a temperature sensitive element to provide an automatic alarm system for excessive temperature or fire in the vehicle.

It is, therefore, an object in making this invention to provide an automatic warning or alarm system.

It is a further object in making this invention to provide an automatic temperature warning system for an automotive vehicle which utilizes one of the components of the vehicle without disturbing normal operation of the same.

It is a still further object in making this invention to provide a fire alarm system for an automotive vehicle which will create an audible signal from the horn in the event of excessive temperature even if the vehicle is unattended.

With these and other objects in view which will become apparent as the specification proceeds, our invention will be best understood by reference to the following specification and claims and the illustrations in the accompanying drawings, in which:

FIG. 1 shows a conventional relay control horn circuit;

FIG. 2 discloses a P-type gate silicon controlled rectifier horn control circuit embodying our invention;

FIG. 3 shows a similar circuit utilizing an N-type gate silicon controlled rectifier for control;

FIG. 4 shows a combination thermistor and P-gate silicon controlled rectifier circuit for a warning system;

FIG. 5 shows a combination thermistor and N-gate silicon controlled rectifier control circuit;

FIG. 6 is a graph showing the V or break over voltage of a silicon controlled rectifier plotted against temperature; and,

FIG. 7 is a graph showing the resistivity of a thermistor plotted against temperature.

In a conventional horn system as currently applied to automotive vehicles the positive power line of the vehicle, such as 2, is connected to a stationary relay terminal 4 of a relay 12 and a second spaced statitonary terminal 6 is directly connected to one terminal of the horn 8, the opposite horn terminal being grounded through line '10. The relay 12 is also provided with an 3',5 l 7,3 Patented June 23, 1970 operating coil 14, one terminal of which is connected to line 2 and the opposite end to stationary terminal 16 of a manually operable switch 18 which may be the horn button. Energization of the relay coil 14 attracts the armature 20 which closes the circuit between contacts 4 and 6 completing a power circuit to the horn and causing the same to be energized or blow. It is to be noted that in this instance the full battery voltage appears across the contacts of the horn button 18-16 and, therefore, making and breaking this circuit may cause these contacts to be burned and occasionally it is necessary for the switch to be replaced.

Instead of a relay it is possible to use a semiconductor device such as a silicon controlled rectifier (SCR) in series with the horn to control the flow of current therethrough to energize and de-energize the horn. Such a circuit is shown in FIG. 2 in which case the power line 2 is connected to the anode 22 of the silicon controlled rectifier 21, the cathode 24 being connected to one terminal of the horn 8, the other terminal of said horn as before being connected to grounded line 10. In this case the silicon controlled rectifier 21 is of the P-gate type so the gate or controlled electrode 26 is connected through a limiting resistor 28 to the movable member 18 of the horn button, the stationary contact 16 of which is connected to the high voltage line 2. In this instance there is normally insufficient conductance through the silicon controlled rectifier 21 to cause the horn to blow. oHwever, when the operator presses the horn button the switch 18-16 closes to apply a control signal to the gate 26 increasing the conductance of the silicon controlled rectifier 21, permitting conduction therethrough and the horn will operate. This silicon controlled rectifier device 21, however, is temperature sensitive and while there is insufiicient flow therethrough at normal temperatures, nevertheless when the temperature becomes sufficiently high that the breakover voltage V decreases to a value smaller than the supply voltage provided by power line 2, as shown in FIG. 6, the device will conduct, permitting the horn to blow even though the switch 16-18 is not closed; and a warning will be issued that the ambient temperature has exceeded a certain predetermined value.

The circuit shown in FIG. 3 is the same as that shown in FIG. 2 except'that the silicon controlled rectifier 30 used therein is of the N-gate type and, therefore, shows the gate associated with the anode instead of the cathode and the resistance and series horn button circuit connected to the ground line 10 instead of the power line 2. The operation is exactly the same.

Thus, by simply utilizing a silicon controlled rectifier to control the horn, two advantages are obtained. First, the control current handled by the switch 18-16 is considerably reduced to a very small value so that there is substantially no burning of the contacts and this switch will last for a long time, together with the fact that an automatic fire alarm is incorporated due to the fact that if the ambient temperature exceeds some value of over C. the conductance will increase to a point where the horn will operate and a warning will be issued to any people in the vicinity of this fact whether the vehicle is attended or not.

FIGS. 4 and 5 show a slightly more sophisticated control circuit for both P-gate and N-gate type silicon controlled rectifier controls. In addition, a thermistor 32 is connected in parallel with the horn button 16-18 of FIG. 4 and its series resistor 28. The P-gate silicon controlled rectifier 21 in this case operates to control the horn 8 in the same manner as that described in FIG. 2 but in addition the control circuit to the gate 26 has incorporated therein the thermistor 32 and this doubly insures control of the silicon controlled rectifier 21 as the temperature rises. Reference to FIG. 7 indicates the change in resistance of the thermistor 32 as the temperature rises. Thus, not only does the silicon controlled rectifier tend to become more conductive as the temperature goes up, but the thermistor produces a control signal on the gate which causes the SCR to trigger to its conductive state and, therefore, bothefiects tend to make sure that the alarm is actuated.

FIG. is similar to FIG. 3 in that the silicon controlled rectifier 30 shown therein is the N-gate type and a therimstor 34 is placed in shunt in this case with the horn button 18-16 between the gate and the ground line,

Thus by improving the control system for normally controlling the horn of the car, we have obtained an automatic fire alarm which warns of impending danger due to excessively high ambient'temperatures.

What is claimed is:

1. In a horn circuit for an automotive vehicle, a source of electrical energy, a conventional vehicle horn operable by electrical energy, a silicon controlled rectifier having a gate control electrode, the vehicle horn and the silicon controlled rectifier being connected in series across the source of electrical energy, the silicon controlled rectifier having such conductance that at normal ambient temperature there is insuflicient current flow therethrough to energize the vehicle horn but at elevated temperatures in the region causing combustion the conductance thereof increases to allow sufiicient current to flow therethrough to energize the vehicle horn to produce an audible warning signal, a series circuit including a resistor and a normally open switch connected between the source of electrical energy and the gate control electrode to cause the silicon controlled rectifier to conduct when the switch is closed to energize the vehicle horn at will, and a thermistor connected in shunt to the resistor and the normally open switch to change the voltage on the gate control electrode of the silicon controlled rectifier upon a predetermined increase in ambient temperature to cause the silicon controlled rectifier to conduct to energize the vehicle horn.

2. In a horn circuit for an automotive vehicle, a voltage source for providing a substantially constant supply voltage, a conventional vehicle horn for producing an audible warning signal when energized, a silicon controlled rectifier having a gate control electrode, the vehicle horn and the silicon controlled rectifier being connected in series across the voltage source, the silicon controlled rectifier having a temperature responsive conductance characteristic such that when the ambient temperature is below a value of approximately C. the breakover voltage of the silicon controlled rectifier is above the supply voltage so that the current flow therethrough is insufiicient to energize the horn but when the ambient temperature increases above a value of approximately 150 C. the breakover voltage of the silicon controlled rectifier decreases below the supply voltage so that the current flow therethrough is sufiicient to energize the vehicle horn, a normally open manually operable switch coupled in series with the voltage source and the gate control electrode so that when the switch is closed a voltage is applied to the gate control electrode to cause the silicon controlled rectifier to conduct thereby energizing the vehicle horn, and a thermistor coupled in series with the voltage source and the gate control electrode so that a voltage is applied to the gate control electrode to cause the silicon controlled rectifier to conduct thereby energizing the vehicle horn only when the ambient temperature increases above a value of approximately 150 C.

References Cited UNITED STATES PATENTS 3,284,787 1/1966 Voigt et a1. 340-227 2,935,730 5/1960 Procter 340-63 3,189,759 6/1965 Laishley.

3,280,392 10/1966 Benda 307-305 X JOHN W. CALDWELL, Primary Examiner D. L. TRAFTON, Assistant Examiner US. Cl. X.R. 

